This may be a real basic question but here goes.

With the Elinchrom BXRi500's there is a digital readout on the back for the power output (2.5, 3.4, etc.). The manual states the watt power of this value but how can this value be related to the f-stop that is so often referenced on this forum for the lights? How can I relate this reading to f9 for instance?

I do understand distance comes into place, like 2.8 goes to 4 if the light backups up 4ft I think.

Thanks in advance and if I have overlooked a link for this in the stickies I apologize.